1. Technical Field
The present invention generally relates to fuel cell stacks, and particularly to methods, apparatus and articles for testing fuel cells and fuel cell stacks prior to operation.
2. Description of the Related Art
Electrochemical fuel cells convert fuel and oxidant to electricity. Solid polymer electrochemical fuel cells generally employ a membrane electrode assembly (xe2x80x9cMEAxe2x80x9d) which includes an ion exchange membrane or solid polymer electrolyte disposed between two electrodes typically comprising a layer of porous, electrically conductive sheet material, such as carbon fiber paper or carbon cloth. The MEA includes a layer of catalyst, typically in the form of finely comminuted platinum, at each membrane/electrode interface to induce the desired electrochemical reaction. In operation the electrodes are electrically coupled to provide a circuit for conducting electrons between the electrodes through an external circuit. Typically, a number of MEAs are serially coupled electrically to form a fuel cell stack having a desired power output.
In typical fuel cells, the MEA is disposed between two electrically conductive fluid flow field plates or separator plates. Fluid flow field plates have at least one flow passage formed in at least one of the major planar surfaces thereof. The flow passages direct the fuel and oxidant to the respective electrodes, namely, the anode on the fuel side and the cathode on the oxidant side. The fluid flow field plates act as current collectors, provide support for the electrodes, provide access channels for the fuel and oxidant, and provide channels for the removal of reaction products, such as water, formed during operation of the cell.
Defects in the ion exchange membrane, such as stray carbon fibers extending from the membrane, can create an electrical short or a potential for an electrical short across the MEA. To detect shorts or the potential for shorting, manufacturers perform stack resistance testing before completing manufacture or shipping to distributors or customers. Manufacturers typically test each fuel cell in each stack, one fuel cell at a time. The stack resistance testing is performed on non-operating stacks, in contrast to other testing or monitoring performed during stack operation. For example laid open, Japanese patent application JP63-117277 teaches applying a direct current to one of the electrodes of a fuel cell and measuring a corresponding potential generated between the two electrodes of the fuel cell. The level and pattern of the voltage response is compared to a level and pattern of voltage response for a xe2x80x9cnormallyxe2x80x9d operating fuel cell.
Existing testing techniques are labor and time intensive. For example, in existing tests a probe is manually repositioned to successively contact each of the fuel cells in the stack. Testing currently takes approximately 45 seconds per fuel cell, and up to 30 minutes to test a stack of 47 fuel cells. Existing testing methods and apparatus result in lower production output and higher costs. Consequently, there is a need for improved methods and apparatus for automated, nondestructive testing of fuel cells and/or fuel cell stacks, particularly for reducing the cycle time required to test fuel cell stacks during manufacturing.
Applicants have recognized that some of the time currently required for testing is attributable to the relatively long delay between applying an input voltage or current and the fuel cell reaching a steady state condition when a resulting output current or voltage is measured, the delay resulting from the inherent capacitive effect of the MEA structure. Applicants have also recognized that some of the time currently required for testing is attributable to the manual repositioning of the probe. Applicants have further recognized that some of the time currently required for testing is attributable to limits on the amount of current or voltage that can be applied to the fuel cell without causing damage to the MEA, for example damage caused by oxidation of the carbon in the electrodes or of catalyst components, including ruthenium (where present) and the carbon of carbon-supported catalysts.
According to one aspect of the invention, a fuel cell resistance test system includes a voltage source selectively operable to produce a defined voltage; a contact head having at least three electrical contacts; means for applying the defined voltage produced by the voltage source successively across pairs of adjacent ones of the electrical contacts; a current sensor coupled to the electrical contacts to measure a resulting supply current; and a processor coupled to receive signals corresponding to at least one of the magnitude of the defined voltage and the magnitude the resulting current, where the processor is configured to determine whether a short exists based on the magnitude of the defined voltage and the magnitude of the resulting current.
According to another aspect of the invention, a fuel cell resistance test system includes a contact head having a plurality of spaced electrical contacts; a plurality of switches, each of the switches selectively actuable to produce a short between a respective pair of adjacent ones of the electrical contacts; a voltage source selectively operable to produce a defined voltage; a processor coupled to open each of the switches, one at a time in succession, to apply the defined voltage from the voltage source successively across pairs of adjacent ones of the electrical contacts; and at least one current sensor coupled to the electrical contacts to measure a resulting current; where the processor is coupled to receive signals corresponding to the magnitude of the defined voltage and the magnitude the resulting current, and configured to determine whether a short exists based on the magnitude of the defined voltage and the magnitude of the resulting current.
In another aspect, a method of testing fuel cell stacks includes simultaneously coupling a plurality of spaced electrical contacts to respective portions of a fuel cell stack; successively applying a defined voltage between each respective pair of adjacent ones of the electrical contacts; successively measuring a respective current resulting from each of the applied defined voltages; and determining whether a short exists based on the defined voltages and the resulting currents.
In a further aspect, a method of testing fuel cell stacks includes simultaneously coupling a plurality of spaced electrical contacts to respective portions of a fuel cell stack; successively applying a defined voltage between each respective pair of adjacent ones of the electrical contacts; successively measuring a respective current resulting from each of the applied defined voltages; and determining whether a short exists based on the defined voltages and the resulting currents.
According to an alternative aspect of the invention, a fuel cell resistance test system includes a current source operable to produce a defined current; a contact head having plurality of pairs of electrical contacts; means for applying a defined current through successive ones of the pairs of electrical contacts while grounding at least some of the other pairs of electrical contacts; at least one voltage sensor to measure a resulting voltage across adjacent ones of the pairs of electrical contacts; and a processor coupled to receive signals corresponding to at least one of a magnitude of the defined current and a magnitude the resulting voltage, the processor configured to determine whether a short exists based on the magnitude of the defined current and the magnitude of the resulting voltage.
In yet a further alternative aspect, a method of testing fuel cell stacks includes simultaneously coupling a plurality of spaced pairs of electrical contacts to respective portions of a fuel cell stack; successively applying a defined current through each respective pair of the electrical contacts; measuring a respective voltage across resulting from each of the applied defined voltages across adjacent ones of the pairs of electrical contacts; and determining whether a short exists based on the defined currents and the resulting voltages.